Today's orientation towards technologies that protect the environment is distinguished by ever more stringent regulations governing the maintenance of air purity. For this reason, attempts are made to use these technologies when coating wood and wood based products. Up to the present time, powder-paint technology has not played any significant role when substrates of this kind are being coated at an industrial scale. Reviews of this subject matter can be found in the following publications: Alfred Kruger, Magdeburg, Josef Heinskill, Brunswick "Moglichkeiten der Pulverbeschich-tung von Holz und Holzwerkstoffen," [Possibilities for Powder-coating, Wood and Wood Products], DFO Tagungsbericht [DFO Conference Proceedings], 1994; Istvan Radoczy, Tigerwerk A-Wels "Das Wood-Dual Verfahren zur MDF-Veredelung," [The Wood-Dual Process for MDF Processing], I-Lack, March 1995, pp. 96-98; Danilo Malavolti "Polveri nella verniciatura del legno e deli plastici?" [Powders for Finishing Wood and Plastics?,"La Revista del Colore/Verniciatura Industriale [Industrial Colours and Finishes Review], No. 281, September 1991, pp. 277-278, Simon Peter Holdmoser, Hali Buromobel, A-Eferding "Pulverbeschichtung von MDF Platten in der Praxis," [Powder Coating of MDF Panels in Practice], JOT Seminarunterlagen [JOT Seminar Papers], Munich, January 1995; G. Sauerbrey, "Holzbeschichtung im Aufbruch," [Wood Coating Flourishing], Report of Powder Meeting, 1991, in CC Hamburg, Vinzenz Verlag, Hannover (Abb. TB Pulver 91); Anon. "Powder coating of wood and woodbased materials," DTI Surface Coating Technology, August, 1993; Leonardo Zelaschi "Novita dal mondo dei vernicianti e della verniciatura," [New from the world of finishes and finishing], La Revista del Colore/Verniciatura Industriale [Industrial Colours and Finishes Review], No. 241, May, 1998, pp. 165-166.
The difficulties that have been encountered up to the present can be attributed to a number of technical causes. Wood is a natural, non-homogeneous and hygroscopic material. Its properties and features are formed by a number of factors that influence it during its growth and when it is being processed. As an organic material, it is constantly being affected by the environment as it is being processed and used.
The substances that make up wood, including moisture that is contained in it, play a decisive role. As the moisture content of wood changes, not only does it volume change--with anistropy exacerbating existing anatomical problems--its electrical conductivity and its behaviour when wet also change.
Different rates of swelling and shrinkage in materials and coatings, which change as tile moisture content changes, impose enormous mechanical stresses on the bonding systems and lead to premature failures at critical points. The constituent substances of the wood migrate to the surface and can result in pronounced colour changes and uncontrolled chemical reactions with the substances used in the coatings.
Problems encountered when powder-coating wood and wood based products are caused by the migration of volatile substances such as water, resins, waxes and terpenes from the substrate; this migration is brought about by the high temperatures that are required in order to form a film from the powder paint. These volatile components result in bubbles being included in the film These, in their turn, degrade the chemical stability of the film, its mechanical properties, and the appearance of the surface of the paint. The lowest possible melt and polymerization temperature is required at the lowest possible viscosity of the molten powder paint in order to reduce off-gassing of the substrate, and in order to encourage de-aeration of the melt. But limits are imposed on these requirements with respect to the shelf life and amenability to processing of powder paints. The exacting demands of the furniture industry for flawless and homogeneous surfaces constitute another challenge to the performance of powder paints of this kind.
For all of the above reasons, it is impossible to simply transfer the technological conditions for powder paint that proved themselves when used such paints are applied to metallic surfaces.
For this reason, I. Radoczy, I-Lack 3/95, 63rd year, pp. 96 et seq. proposes using a symbiosis of powder paint and water-based paint. The author proposes a formulation for a water paint that has been adapted for wood, and which is used to wet the substrate. Using this water paint causes the powder paint to adhere to the wood substrate. However, a disadvantage with this procedure is that the advantages of the totally emission-free dry powder paint are not exploited. For this reason, there is still a need to use powder paints without the addition of water-based paints.
For this reason, it is the task of the present invention to make available a layered material consisting of a substrate that is of heat-sensitive material, preferably wood, and a powder paint coat that is applied thereto, which simultaneously ensures the minimal formation of bubbles and optimization of its flow and polymerization characteristics.